This invention concerns a photometer comprising a photo diode, arranged to produce an electric current proportional to the intensity of light picked up, the photo diode being linked by the intermediate of a resistance to an operational amplifier equipped with a band-pass filter.
Such a photometer is known from U.S. Pat. No. 3,790,288. The photometer is used for determining the intensity of the emitted light by an incident light source. For this purpose it includes a photo diode, generally of silicium, which converts the incident light into electric current. An operational amplifier allows supplying to its exit a tension that indicates the intensity of the light picked up. In the known device, the resistance connected between the photo diode and an entry of the operational amplifier, and another resistance, both present in the regulation loop of the amplifier, together determine the amplification factor of the loop.
A disadvantage of the known photometer is that the measuring precision is too limited for certain applications where a precision of 0.01% in the variation of intensity of the light is required.
The invention has the purpose to realize a photometer allowing to obtain a precision of 0.01% or less.
For this purpose a photometer according to the invention is characterized in that the value of the resistance is sized in such a way as to level the peak of resonance in the pass-band of the filter. The invention is based on the constatation that the peak of resonance, present in the pass-band of the filter, which forms the regulation loop of the amplifier, sensibly limits the precision of the photometer, because it prevents all measurements in a frequency band around this peak of resonance. Sizing the resistance between the photo diode and the amplifier in such a way as to level the peak, opens this frequency band to a precise measurement of the intensity of light, because the gain of the amplifier is now practically constant in the whole pass-band.
It is necessary to point out that, even if the use of a resistance at the entry of an operational amplifier is known in itself, and in particular in disclosed prior art, as a component, allowing to determine the factor of amplification of the loop, nothing in the prior art allows the use of this resistance as an electronic component, allowing to absorb the energy of the resonance peak in the pass-band.
A first preferential embodiment of a photometer according to the invention is characterized in that for a resonance peak of about 2.5 KHZ, the value of the resistance is situated between 1 Mxcexa9 and 1.6 Mxcexa9. This allows to maintain the gain of the amplifier sensibly constant in the most usual pass-band for a photometer.
A second preferential embodiment of a photometer according to the invention is characterized in that the amplifier includes an exit, arranged for providing there a proportional electric tension signal to said electric current, said exit being linked to an analog-digital converter, the latter linked to a sampling unit, arranged for sampling said tension signal and furnishing a set of samples. This allows digital computer treatment of the signal.
Preferably, the unit of sampler is linked to an accumulator, arranged for accumulating a predetermined number of samples and for determining their average. This allows to reduce the electric noise.